Age-dependent dysfunction in adult stem cells is attributable to both cell-intrinsic and -extrinsic inputs. Critical mechanisms underlying the functional decline of aged stem cells remain elusive. Accordingly, there exists a need to identify factors that are able to promote or reverse age-associated changes in tissues as diverse as the skeletal muscle, liver and CNS (Wagers and Conboy, Cell 2005; 122, 659; Ruckh et al. Cell Stem Cell 2005; 10, 96).